Biomass Combustion

ABSTRACT

A splitter divides a flow of low heating value biomass into a central stream and an annular stream. A stable flame may be achieved by combusting the central stream with oxygen. This avoids the use of costly fossil fuels or biomass (that have higher heating values than the biomass fuel) as an auxiliary fuel for achieving a stable flame.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/616,245, filed Mar. 27, 2012 and U.S. Provisional Application No.61/619,905, filed Apr. 3, 2012.

BACKGROUND

1. Field of the Invention

The present invention relates to biomass burners and methods ofcombusting biomass.

2. Related Art

The emission of carbon dioxide (CO₂) as a cause of global warming is ofcurrent concern to the power industry. The potential role of biomassenergy acquired a new dimension when it was suggested that plantinglarge areas of new forest could slow the increase in atmospheric carbondioxide by removing carbon dioxide from the atmosphere. Therefore, theelectric power industry uses biomass in order to significantly reduceCO₂ emissions.

In a typical staged combustion burner firing biomass, the biomass fuelis combusted with primary combustion air in a first combustion zone. Anynon-combusted fuel is more completely combusted in a second combustionzone downstream of the first combustion zone with secondary combustionair injected around the biomass. If tertiary combustion air is utilized,combustion is completed in a third combustion zone downstream of thesecond combustion zone with tertiary combustion air injected around thesecondary combustion air.

The heating value of widely available biomass fuels is generally lowerthan that of fossil fuels. In order to establish a stable flame in theboiler, a fossil fuel having a higher heating value than the biomass istypically co-fired with the biomass at the burner to ignite the flame.The combustion of the fossil fuel with available oxidant provides thenecessary energy to ignite the biomass fuel. Usually, the fossil fuel iscoal, oil, or natural gas in order to ensure the flame ignition andstability. Instead of using fossil fuel, another option is to injecthigher heating value biomass, such as rapeseed oil, with the lowerheating value biomass fuel for purposes of flame ignition. Again, thisadditional biomass has a higher heating value than the main biomass fueland is used in the same manner as the fossil fuel in the above-describedprocesses.

If an existing furnace designed for combusting natural gas isretrofitted for firing biomass fuel, such a retrofit has the potentialof limiting the apparent power of the burner. This is because the solidbiomass particles are combusted more slowly than a gaseous fuel. Atlower furnace loads, the biomass burner is able to inject a flow ofbiomass and satisfactorily burn out the biomass particles before theyimpinge a furnace wall opposite the burner. At higher furnace loads,however, a higher velocity of primary combustion air will becomenecessary to achieve satisfactory conveyance of the biomass particles sothat they may be injected by the burner. If a higher velocity of primarycombustion was not used, the otherwise low momentum of the biomassparticles would cause them to settle and accumulate. The higher velocityof the combustion air and biomass particles results in a residence timefor the particles in between the burner and the opposing furnace wallthat is too short to allow satisfactory burn-out of the biomassparticles. Thus, although the biomass fuel flow rate may be increased,the resultant steam power from the boiler may be limited due to lessthan complete combustion of the biomass particles and inefficient heattransfer from the combustion of the biomass particles to the boilersteam tubes. In other words, the apparent burner power may be limited.

There has been several biomass combustion processes proposed in thepatent literature.

U.S. Pat. No. 5,107,777 describes combustion of a low BTU high moisturebiomass such as wood (known as Hog fuel). Biomass is injected into theboiler 15-20 ft above the floor. The combustion air, which is suppliedfrom the bottom of the furnace is enriched with oxygen to a level ofbetween 0.1 to 7%. Additional oxygen is also injected from the side. Oilburners are fired from the top. It claims that a higher flametemperature is achieved with injection of oxygen.

US 2008/0261161 A1 describes a burner or furnace for the combustion ofbiomass using two or more fuel injection ports located at non-radialinjection angles. The biomass is mixed with oxidizer and then injectedinto the furnace via a cyclonic combustion vortex.

U.S. Pat. No. 6,699,029 B2 describes a boiler system where a low rankfuel is burned to achieve energy generation rate similar to thatachieved with conventional fuels such as coal. It proposes certainoxygen injection methods for reducing the formation of nitrogen oxides(NOx). Operations with typical US-origin coals are described.

The co-firing system described above has been adopted in many EUelectric power plants to meet the tightening EU regulations. While theco-firing or central injection of a higher heating value fossil orhigher heating value biomass in the combustion of the lower heatingvalue main biomass fuel may keep the flame ignited and provide a stableflame, the cost of the higher heating value biomass fuel or fossil fuelis very expensive compared with generally available lower heating valuebiomass such as wood and straw. Also, the current conventional co-firingsystem is relatively complex because it includes two fuel feedingsystems.

Thus, it is an object of the invention to provide a burner, combustionsystem and method of combustion that would avoid or reduce the usage ofthe fossil fuel/high quality biomass and reduce the capital cost andmaintenance cost while achieving a stable flame and maintaining ignitionof the flame.

It is also an object of the invention to provide a burner, combustionsystem, and method of combustion that would tend to remove thelimitation on the apparent power of the burner as the biomass fuel flowrate is increased.

SUMMARY

There is disclosed a biomass burner, comprising: a burner block, a fuelconduit, an oxygen injector, and a tubular fuel flow splitter. Theburner block has an injector passage extending between rear and frontfaces. The fuel conduit has inlet and outlet ends and is concentricallydisposed within said bore at said front face, an annular combustion airflow space being defined between an inner surface of said outer conduitand an outer surface of said fuel conduit. The oxygen injector has inletand outlet ends and is concentrically disposed within said outer conduitat said front face. The tubular fuel flow splitter is concentricallydisposed within said fuel conduit at said front face. The splitter hasan inlet end disposed upstream of said fuel conduit inlet end and alsohas an outlet end. The oxygen injector has either an annularcross-sectional shape and is adjacent to and surrounds said splitter, ora cylindrical cross-sectional shape and is concentrically disposedwithin said splitter.

There is also disclosed a biomass combustion system, comprising theabove-disclosed biomass burner, a biomass hopper, a biomass fuel feeder,a source of oxygen, and one or more blowers. The biomass fuel feeder isoperatively associated with said hopper and at least one of said one ormore blowers to receive particulate biomass from said hopper, convey theparticulate biomass with a flow of combustion air from said at least oneblower to provide a flow of biomass fuel, and direct the flow of biomassfuel to said fuel conduit inlet end. At least one of said one or moreblowers is in fluid communication with said combustion air flow space.The source of oxygen is in fluid communication with said oxygen injectorinlet end.

There is also disclosed a biomass-fired boiler installation, comprising:a plurality of the above-disclosed biomass burner; one or more blowers;at least one biomass hopper; at least one biomass fuel feeder; a sourceof oxygen; and a boiler. Said at least one biomass fuel feeder isoperatively associated with said at least one hopper and at least one ofsaid one or more blowers to receive particulate biomass from said atleast one hopper, convey the particulate biomass with a flow of air fromsaid at least one blower to provide a flow of biomass fuel, and directthe flow of biomass fuel to said fuel conduit inlet ends. At least oneof said at least one blower is in fluid communication with saidcombustion air flow spaces. Said source of oxygen is in fluidcommunication with said oxygen injector inlet ends. Said plurality ofburners is mounted on walls of said boiler.

There is also disclosed a method of combusting biomass, comprising thefollowing steps. A flow of particulate biomass conveyed with air from afuel conduit of a biomass burner is injected into a combustion space. Aflow of oxygen is injected into the flow of injected biomass from anoxygen injector concentrically disposed within said fuel conduit. Theinjected central flow of biomass is combusted with the oxygen in thecombustion space. An annular flow of combustion air is injected from theburner around the annular flow of biomass. The injected annular flow ofbiomass is combusted with the combustion air in the combustion space.The fuel conduit has a tubular splitter concentrically disposed therein.The flow of biomass is split by the splitter into a central flow on theinside of the splitter and an annular flow on the outside of thesplitter.

There is also disclosed a method of retrofitting a conventionalbiomass-fired boiler installation. The boiler installation comprises: aplurality of biomass burners designed for combusting biomass only withair; one or more blowers; at least one biomass hopper; at least onebiomass fuel feeder; and a boiler. Said at least one biomass fuel feederis operatively associated with said at least one hopper and at least oneof said one or more blowers to receive particulate biomass from said atleast one hopper, convey the particulate biomass with a flow of air fromsaid at least one blower to provide a flow of biomass fuel, and directthe flow of biomass fuel to said fuel conduit inlet ends. At least oneof said at least one blower is in fluid communication with saidcombustion air flow spaces. Said plurality of burners is mounted onwalls of said boiler. Said method comprises the steps of: replacing oneor more of the burners designed for air-combustion with a correspondingnumber of the above-disclosed inventive burners and placing a source ofoxygen in fluid communication with said oxygen injector inlet ends.

Any of the above-disclosed burner, biomass combustion system,biomass-fired boiler installation, method of combusting biomass, andmethod of retrofitting a conventional biomass-fired boiler installationmay include one or more of the following aspects:

-   -   the outlet ends of the fuel conduit, oxygen injector, and        splitter are flush with said front face.    -   the outlet ends of said oxygen injector and splitter are        recessed back from said fuel conduit outlet end.    -   said oxygen injector has an annular cross-sectional shape and is        adjacent to and surrounds said splitter.    -   said oxygen injector outlet end is configured as a closed face        with a plurality of radially distributed injection holes.    -   said oxygen injector outlet end is configured as an open face.    -   said oxygen injector has a cylindrical cross-sectional shape and        is concentrically disposed within said splitter.    -   said oxygen injector outlet end is configured as a closed face        with a plurality of radially distributed injection holes.    -   said oxygen injector outlet end is configured as an open tube.    -   the burner further comprises an outer conduit concentrically        disposed within said bore having an inlet end disposed        downstream of said burner block rear face and also having an        outlet end, said annular combustion air flow space being split        into a secondary combustion air flow space and a tertiary        combustion flow space by said outer conduit, the secondary        combustion air flow space being defined by an outer surface of        said fuel conduit and an inner surface of said outer conduit,        and the tertiary combustion air flow space being defined by an        outer surface of said outer conduit and an inner surface of said        bore.    -   the biomass burner further comprises a secondary combustion air        swirler disposed within said secondary combustion air flow space        upstream of said burner block front face, and a tertiary        combustion air swirler disposed along an inner surface of said        bore adjacent to said burner block front face.    -   said splitter further comprises a main section extending between        said splitter inlet and outlet ends, the splitter inlet end        having a diameter D1, the main body having a diameter D2,        wherein D1<D2.    -   said fuel conduit has a diameter D4, said splitter inlet end has        a diameter D1 and 0.05 D4≦D1≦0.25 D4.    -   said source of oxygen is selected from group consisting of a        vacuum swing adsorption system, an oxygen pipeline, a cryogenic        air separation unit, and a vaporizer connected to a tank of        liquid oxygen.    -   said source of oxygen is selected from group consisting of a        vacuum swing adsorption system, an oxygen pipeline, a cryogenic        air separation unit, and a vaporizer connected to a tank of        liquid oxygen.    -   the central biomass flow has a velocity V1, the annular biomass        flow has a velocity V2, and the flow of oxygen has a velocity        V3, where (V3−V2)<(V3−V1).    -   no fuel other than the particulate biomass is combusted.    -   the oxygen and the central flow of biomass begin to mix at a        point upstream of said fuel conduit outlet end.    -   said oxygen is injected in a center of the central flow of        biomass.    -   said oxygen is injected in an annulus surrounding said splitter.    -   the oxygen is swirled.    -   the oxygen has a concentration of >95%.    -   the combustion air is injected in two annular flows, a first of        which is secondary combustion air adjacent the annular flow of        biomass and a second of which is tertiary combustion air        adjacent the secondary combustion air.    -   an overall oxygen enrichment of the combined biomass fuel,        injected oxygen and combustion air achieved by injection of the        oxygen is between 21% and 25%.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1A is a schematic, cross-sectional view of an embodiment of theinventive burner.

FIG. 1B is a variation of the burner of FIG. 1A.

FIG. 1C is a variation of the burner of FIG. 1A.

FIG. 1D is a variation of the burner of FIG. 1C.

FIG. 2A is a front elevation view of the oxygen lance of the burners ofFIG. 1A or FIG. 1B.

FIG. 2B is a front elevation view of an oxygen nozzle for use with theburners of FIG. 1C or FIG. 1D.

FIG. 2C is a front elevation view of another type of oxygen nozzle foruse with the burners of FIG. 1C or FIG. 1D.

FIG. 3A is a schematic, front elevation view of a simulated burner ofthe Comparative Example.

FIG. 3B is a partial, cross-sectional view of the burner of FIG. 3Ataken along axis X-X that illustrates streams of rapeseed oil, woodpellets, and secondary combustion air.

FIG. 4A is a schematic, front elevation view of a simulated burner ofExamples 1-8.

FIG. 4B is a partial, cross-sectional view of the burner of FIG. 4Ataken along axis Y-Y that illustrates streams of oxygen, wood pellets,and secondary combustion air.

FIG. 5 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by the Comparative Example.

FIG. 6 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 1.

FIG. 7 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 2.

FIG. 8 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 3.

FIG. 9 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 4.

FIG. 10 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 5.

FIG. 11 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 6.

FIG. 12 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 7.

FIG. 13 is a graph showing the temperature distribution of the simulatedflame and combustion chamber yielded by Example 8.

DESCRIPTION OF PREFERRED EMBODIMENTS

The proposed invention solves the problem experienced by conventionalbiomass combustion by instead combusting a portion of lower heatingvalue biomass injected from a central portion of the burner with oxygento establish a stable flame ignition. The invention avoids the necessityof having a second fuel feeding system. It also reduces operationalcosts by avoiding or at least reducing the use of the relativelyexpensive higher heating value fuels. It also improves the ability ofthe burner to be boosted to a higher apparent burner power.

The overall flow of lower heating value biomass fuel in a fuel conduitis split into an inner, central stream and an outer, annular stream by asplitter. Upstream of the splitter, the velocities of each portion ofthe flow of biomass fuel are generally uniform. Because the narrowerdiameter splitter inside the fuel conduit creates a pressure drop, thevelocity of the portion of biomass fuel entering the stream is lowered.On the other hand, there is little to no change in velocity of theportion of biomass that flows on the outside of the splitter. Oxygen isinjected into the inner biomass stream from an injector. The injectormay be concentrically disposed within the splitter or may be adjacent toand surrounds the splitter. The velocity of the injected oxygen ishigher than each of the inner and annular streams of biomass. Becausethe difference in velocities between the oxygen and inner stream isgreater than the difference in velocities between the oxygen and theannular stream, there is relatively more mixing between the inner streamand the injected oxygen in comparison to the annular stream and oxygen.

A stable central flame rooted near the face of the burner is achievedbecause the locally high concentration of oxygen in the mixedoxygen/inner stream allows the biomass particles of the inner stream tobe ignited more easily and at an earlier point than the biomassparticles in the annular stream. Thus, a combustion reaction iscommenced at an upstream, inner zone adjacent the burner face betweenthe biomass particles of the annular stream and the oxygen from both thebiomass conveying air and also the injected oxygen.

A stream of combustion air is injected by the burner around the annularflow of biomass. A combustion reaction is commenced between the streamof combustion air and the biomass particles of the annular stream at adownstream, annular zone farther away from the burner face. The biomassparticles of the annular stream are ignited less easily and at a laterpoint than those of the inner stream because of the relatively loweroxygen concentration of the combustion air in comparison to the mixedoxygen and inner stream of biomass. There is little to no oxygen fromthe injected stream of oxygen available to combust the biomass particlesin the annular stream. This is because there is relatively less mixingbetween the oxygen and the biomass particles in the annular stream andthe injected oxygen is mostly or entirely already consumed throughcombustion with the biomass particles in the inner stream.

The combustion air may be swirled or not. The combustion air may be asingle stream or it may be split into a secondary combustion air streamsurrounding the annular biomass stream and a tertiary combustion airstream surrounding the secondary combustion air stream. Either or bothof the secondary and tertiary combustion air streams may be swirled.

While the biomass fuel may be any biomass fuel known in the art,typically it is wood pellets, straw, or so-called hog fuel.

The oxygen is industrially pure oxygen. The specific purity of theindustrially pure oxygen depends upon the method of production andwhether or not the produced oxygen is further purified. For example, theindustrially pure oxygen may be gaseous oxygen from an air separationunit that cryogenically separates air gases into predominantly oxygenand nitrogen streams in which case the gaseous oxygen has aconcentration exceeding 99% vol/vol. The industrially pure oxygen may beproduced through vaporization of liquid oxygen (which was liquefied fromoxygen from an air separation unit, in which case it, too, has a purityexceeding 99% vol/vol. The industrially pure oxygen may be also beproduced by a vacuum swing adsorption (VSA) unit in which case ittypically has a purity of about 92-93% vol/vol. The industrially pureoxygen may be sourced from any other type of oxygen productiontechnology used in the industrial gas business.

Although high concentration oxygen is used in the center oxy-flame forignition, the overall oxygen enrichment is typically between 21% and25%, where enrichment is defined as:

where:

$\frac{V_{{central}\; O\; 2} + \left( {V_{{combustion}\mspace{14mu} {air}} \cdot 0.209} \right)}{V_{{central}\; O\; 2} + V_{{combustion}\mspace{14mu} {air}}} \times 100\%$

where:

-   -   V_(central O2) is the volumetric flow rate of oxygen in the        centrally injected oxygen    -   V_(combustion air) is the volumetric flow rate of combustion air        Although higher levels of oxygen enrichment may be used,        extremely high levels may produce a flame with a temperature to        melt the ash thereby producing slag. Generally, it is preferred        to avoid slag formation.

The burner includes a burner block that is installed into an opening inthe wall of the furnace, and at least a fuel conduit, a fuel splitterconcentrically disposed within the fuel conduit, and an oxygen injectordisposed within the fuel conduit. The burner block is typically made ofrefractory material and includes a bore through which the fuel conduit,splitter, and oxygen injector extend. If both secondary and tertiarycombustion air streams are desired, an outer conduit may also beincluded in the burner in which case it is disposed concentricallywithin the bore. Each of the fuel conduit, splitter, oxygen injector,and optional outer conduit are typically made of any metal suitable forburner elements.

As best illustrated in FIG. 1A, biomass is conveyed by air through thefuel conduit 1 to provide a fuel stream 11. The fuel stream 11 is splitinto two streams, a central, inner stream 8 and an annular, outer stream9, by the splitter 7. The splitter is a tubular structure that has anarrower inlet end with a diameter D1, a main section having a diameterD2>D1, and an outlet end that has a diameter D3≧D2.

High concentration oxygen is injected through the oxygen lance 2. Theoxygen and the inner stream 8 of the biomass pellet are mixed andignited in the center of the flame. The split ratio (the ratio of themass flow rate of the outer biomass stream 9 to the mass flow rate ofthe inner biomass stream 8) may be adjusted by changing the angle of thethroat of the splitter 7 or by increasing or decreasing D1 for a givenfuel conduit 1 diameter D4. Typically, 95%-50% of the fuel stream 11becomes stream 9 while 5-50% of the fuel stream 11 becomes stream 8.Thus, the diameter D1 of the splitter 7 is related to the diameter D4 ofthe fuel conduit 1 according to the equation: 0.05·D4<D1<0.25·D4.

A stream of combustion air 13 flows in the annular space between thefuel conduit 1 and an inner surface 14 of the bore of the burner blockB. The stream of combustion air 13 is divided into two portions by asliding air damper 4. Secondary combustion air 6 flows in the annularspace in between the fuel conduit 1 and an outer conduit 12, while thetertiary combustion air 5 flows in the annular space between the outerconduit 12 and an inner surface 14 of the bore of the burner block B.Each or either of the secondary and tertiary combustion air streams 6, 5may be swirled. Typically, the secondary combustion air stream 6 isswirled by a swirl generator 10 before enters into the combustion spaceC of the boiler, while the tertiary combustion air stream 5 is swirledat the face of the burner where a plane P divides the combustion space Cfrom the furnace wall W and the burner. The overall air swirl intensitymay be adjusted by manipulation of the sliding air damper 4.

The outer, annular stream 9 of the biomass and the secondary combustionair stream 6 are mixed and ignited by the center oxygen/biomass flame.Combustion of the biomass fuel is completed by the tertiary combustionair stream 5.

The oxygen lance 2 may be configured in any one of several ways known inthe field of oxy-combustion. Thus, the oxygen lance 2 may be a straighttube with a constant diameter. Alternatively, the oxygen lance 2 maydiverge at its outlet end where the oxygen mixes with the inner stream 8of biomass fuel. As best shown in FIG. 2A, the oxygen lance 2 couldinstead include a group of small nozzles 0 evenly distributed at theoutlet end in order to even distribute the oxygen in the surroundinginner biomass fuel stream 8 and enhance mixing of the two. Anotherpossible variation is to swirl the oxygen stream from the oxygen lance2. A swirled oxygen stream will increase the mixing between the innerstream of biomass fuel 8 and the oxygen from the lance 2 and help keepthe flame ignited.

In a variation of the burner of FIG. 1A, and as best shown in FIG. 1B,the oxygen lance 2 and the splitter 7 may be recessed from the plane Pby a distance L. Such a configuration will result in mixing of theoxygen and the biomass in the inner stream 8 to ignite a flame at apoint upstream of the plane P. The outlet ends of the fuel conduit 1,outer conduit 12 and burner block B are still flush with the plane P.

In a variation of the burner of FIG. 1A, and as best illustrated in FIG.1C, instead of injecting oxygen from a position inside the splitter 7,the oxygen could be injected from a nozzle 2″ having an annularcross-section. The nozzle 2″ is adjacent to, and surrounds, the splitter7 so that the inner stream of biomass 8 is combusted with an annularstream of oxygen. The nozzle 2″ is fed by an oxygen conduit 2′.

In a variation of the burner of FIG. 1C, and as best shown in FIG. 1D,the oxygen nozzle 2″ and the splitter 7 may be recessed from the plane Pby a distance L. Such a configuration will result in mixing of theoxygen and the biomass in the inner stream 8 to ignite a flame at apoint upstream of the plane P. The outlet ends of the fuel conduit 1,outer conduit 12 and burner block B are still flush with the plane P.

In either of the burners of FIG. 1C or 1D, the oxygen nozzle 2″ may beconfigured in a couple of different ways. As best illustrated in FIG.2B, the oxygen nozzle 2″ may be open at the outlet end. Alternativelyand as best shown in FIG. 2C, oxygen may be injected from the outlet endof the oxygen nozzle 2″ from a plurality of radially distributed holes2bis.

The furnace may be shut down at regular intervals (i.e., annually) forfurnace maintenance. It may be desirable at the resumption of furnaceoperation to first heat the furnace by combusting a stream of atomizedoil from an oil gun with secondary combustion air in the conventionalmanner. Once a predetermined furnace temperature is reached, injectionof the biomass fuel is initiated. Normal operation of the furnace isthen commenced upon discontinuance of the stream of atomized oil andremoval of the oil gun from the furnace. With this in mind, one ofordinary skill in the art will recognize that, despite the use of such aconventional furnace pre-heating technique, during normal operation theburner and furnace only combusts a single fuel: biomass.

Regardless of whether the oxygen is injected according to the burnerconfigurations of FIG. 1A, 1B, 1C, or 1D, the injection of oxygen intoan inner flow of biomass fuel 8 helps increase burnout of the biomassfuel particles in comparison to conventional biomass burner where nosuch oxygen injection is employed. Burnout is increased because thelocal oxygen concentration surrounding the biomass particles in theinjected inner stream 8 is increased. An oxygen-enriched atmosphere atthis region not only starts combustion of volatile components in thebiomass particles earlier but also starts combustion of char earlier. Asa result, satisfactory burnout of the biomass particles is completed inthe path line of the biomass particles inside the furnace at a pointearlier in comparison to biomass particles from biomass burners where nosuch oxygen injection is performed.

Faster burnout of the biomass particles is advantageous for allowingsatisfactory operation of the biomass burner at higher apparent powers.This will be clearly evident when compared to operation of aconventional biomass burner in which no central oxygen injection isperformed. When conventional biomass burners are operated at lowerpowers, the flow rate of primary combustion air necessary forsatisfactory conveyance of the biomass particles has a velocitysufficiently low that satisfactory burnout of the biomass particles maybe achieved over the path line traveled by the particles through thefurnace. At higher burner powers, the flow rate of primary combustionair that is necessary for satisfactory conveyance of the biomassparticles must be increased because the total mass of solid biomassparticles is increased. As the flow rate of the primary combustion airis increased, it will soon reach a velocity that is too high to allowsatisfactory burnout of the solid biomass particles along the path linethrough the furnace and enter the superheater. In other words, theresidence time of a combusting biomass particle is decreased when highervelocity combustion air is used (such as at higher burner powers). Sucha situation creates several disadvantages.

One disadvantage is related to wear to the furnace. In comparison to therelatively lower combustion air velocities when the burner is operatedat lower power, the relatively higher combustion air velocities athigher burner powers changes the pattern of heat transfer from thecombusting particles to the furnace. More particularly and in comparisonto lower burner powers, relatively less heat is transferred to portionsof the furnace closer to the burners and relatively more heat istransferred to portions of the furnace relatively distant from theburners. This shift in the amount of heat transferred to portions of thefurnace adjacent the superheater can result in damage to that portion ofthe furnace because it is not designed for excessive radiative heattransfer.

The second disadvantage is realized for biomass furnaces that wereoriginally commissioned as coal-fired furnaces but which have beenretrofitted for biomass combustion. Coal-fired furnaces are designed tobe heated by a large number of burners. Together, those burners providea nominal power at which the furnace is designed to operate. The nominalpower is related to the heat flux from combustion of the coal to wateror stream in the boiler steam tubes and which is realized in the form ofmechanical or electrical power. If the furnace is retrofitted withconventional biomass burners, at relatively high biomass fuel firingrates the burners may fall well short of the nominal power due tounsatisfactory burnout of the biomass particles. Primarily, this isbecause typical biomass particles (with an average size of around 100 m)combust more slowly than typical pulverized coal particles (with anaverage size of around 60 m). Although the furnace may have beendesigned to achieve the nominal power with the more quickly combustingcoal particles, the more slowly combusting biomass particles shifts thepattern of heat transfer from the combusting particles to the furnace.In particular, less heat is transferred to portions of the furnaceadjacent to upstream portions of the path line and more heat istransferred to portions of the furnace adjacent to downstream portionsof the path line. Typical furnaces are not designed for such a modifiedheat transfer pattern where much of the heat transfer is shifteddownstream along the path line. So, as the flow rate of the biomass fuelfrom the burner is increased in an attempt to increase the power, theapparent power of the burner soon reaches a limit beyond which it isdifficult to increase by increasing the flow rate of the biomass fuel.

In contrast, by injecting oxygen in an inner flow of biomass fuelaccording to the invention, the above disadvantages may be avoided. Thehigher oxygen concentrations surrounding the biomass particles tends toignite the flame earlier and increases the rate at which the biomassparticles combust. As a result, the impact of the downstream shift inheat transfer that would otherwise be experienced in furnaces fired withconventional biomass burners is reduced or nullified by the increase inthe rate of combustion of the biomass particles afforded by thelocalized oxygen-enriched environment. Because the distribution of heattransfer from the combusting particles to the furnace more closelymatches the distribution of heat transfer that the furnace wasoriginally designed for when it was commissioned as a coal-firedfurnace, the apparent power of the burner may still be increased throughan increase in the flow rate of the biomass fuel from the burner. Also,the above-described increase in furnace wear caused by conventionalbiomass burners is either decreased or avoided.

Thus, the invention provides multiple benefits. The invention canimprove the overall system efficiency with minimum modifications on thecurrent boiler combustion system. It can reduce a power plant's CO₂ footprint. Oxygen enrichment will reduce the flue gas volume. The proposedburner and combustion system only has one fuel and one fuel feedingsystem, so it is reduced in complexity in comparison to conventionalbiomass combustion processes. The avoidance of, or reduction in use of,a higher heating value auxiliary fossil fuel or biomass fuel reduces theoperational cost. Finally, the apparent burner power may be increasedbeyond levels achievable with conventional biomass burners. Excessfurnace wear may be reduced or avoided.

PROPHETIC EXAMPLES

Conventional and inventive biomass combustion processes were simulatedin two dimensions axi-symmetrically with Fluent™ computational fluiddynamics (CFD) software.

Comparative Example

As best illustrated in FIGS. 3A, 3B, a burner was simulated thatincluded a central stream 22 of rapeseed oil droplets (mean diameter of100 μm) conveyed with primary combustion air, an annular stream 24 ofprimary combustion air, an annular stream 26 of wood pellets (meandiameter of 100 μm) conveyed with primary combustion air, and an annularstream 28 of secondary combustion air. The outer edge 29 of thesecondary combustion air stream 28 had a diameter of 6 inches (15.24cm). The outer edge 27 of the outer stream 26 had a diameter of 3 inches(7.62 cm). The outer edge 25 of the inner stream 24 had a diameter of 1inch (2.54 cm). The outer edge 23 of the rapeseed oil stream 22 had adiameter of 0.375 inches (0.9525 cm). Each of the outer stream 26 andthe secondary combustion air stream 28 was swirled with a 45° swirlangle. The rapeseed oil had a heating value of 39,000 kJ/kg and anelemental composition of C_(18.95)H_(35.3)O₂, while the wood pellets hada heating value of 19,700 kJ/kg. The physical and elemental compositionsof the wood pellets are listed in Tables I and II, respective. The massflow rates of the various streams 22, 24, 26, 28 are listed in TableIII.

TABLE I physical composition of wood pellets physical component weightfraction Volatile 0.75 Fixed Carbon 0.13 Ash 0.025 Moisture 0.095

TABLE II elemental composition of wood pellets element weight fraction C0.53128 H 0.05846 O 0.40615 N 0.00380 S 0.00031

TABLE III Mass flow rates of streams component Mass Flow Rate (kg/h)primary combustion air in central stream 22 3.97 rapeseed oil dropletsin central stream 22 0.6923 primary combustion air in annular stream 243.97 wood pellets in annular stream 26 26.5 primary combustion air inannular stream 26 26.5 secondary combustion air in annular stream 28143.7

A “slice” of the burner and streams 22, 24, 26, 28 was taken along axisX-X and simulated with CFD.

EXAMPLES

As best illustrated in FIGS. 4A, 4B, a burner was simulated thatincluded a central stream of oxygen 22 from an oxygen lance 23. Aninner, annular stream 24 of wood pellets (mean diameter of 100 μm)conveyed with primary combustion air was injected from the inside of afuel splitter 25 surrounding the central stream 22. An outer, annularstream 26 of the wood pellets conveyed with primary combustion air wasinjected from in between the splitter 25 and a fuel conduit 27. Finally,an annular stream 28 of secondary combustion air was injected from inbetween the fuel conduit 27 and a bore 29 in a burner block. The bore 29had a diameter of 6 inches (15.24 cm). The fuel conduit 27 had adiameter of 3 inches (7.62 cm). The splitter 25 had a diameter of 1 inch(2.54 cm). The oxygen lance 23 had a diameter of 0.375 inches (0.9525cm). The rapeseed oil and wood pellets are the same as those used forthe Comparative Example. The mass flow rates of the various streams 22,24, 26, 28, the levels of oxygen enrichment, and swirl angles forstreams 26, 28 for Examples 1-9 are listed in Table IV.

TABLE IV mass flow rates and enrichment/swirl parameters for Examples1-9 Mass Flow Rate (kg/h) Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Ex 7 Ex 8component oxygen in 32 1.86 1.86 3.31 1.86 1.86 1.86 3.72 1.86 primarycombustion 1.37 1.37 2.74 1.37 1.37 1.37 1.37 2.74 air in 34 woodpellets in 34 1.37 1.37 2.74 1.37 1.37 1.37 1.37 2.74 primary combustion26.5 26.04 24.67 26.5 26.5 26.5 26.04 24.67 air in 36 wood pellets in 3626.5 26.04 24.67 26.5 26.5 26.5 26.04 24.67 secondary 143.7 135.1 128143.7 143.7 143.7 127.1 135.1 combustion air in 38 parameter O₂enrichment (%) 21.76 21.81 22.49 21.76 21.76 21.76 22.68 21.81 swirlangle of 36 45 45 45 45 63.44 63.44 45 45 swirl angle of 38 45 45 4526.56 45 26.56 45 45

A “slice” of the burner and streams 32, 34, 36, 38 was taken along axisY-Y and simulated with CFD.

Results

The temperature distribution in the flames and combustion chambers forthe Comparative Example and Examples 1-8 are displayed in FIGS. 5-13.The simulation results show that combustion of wood pellets with arelatively small amount of oxygen at the center of the burner canachieve combustion results similar to those achieved with rapeseed oilbut no central oxygen. Also, in the oxy-combustion cases, the resultsshow that the flame shape and temperature profiles can be manipulatedeither by changing the mass flow rates of the different streams or bychanging the swirl angle of the secondary combustion air stream and/orthe swirl angle of the outer, annular stream of wood pellets.

A comparison of Examples 2 and 7 shows that, if the velocity of thecenter O₂ injection is doubled and the mass flow rate of the stream ofsecondary combustion air is decreased by a corresponding amount, theoxygen enriched combustion zone is extended and the flame ignition pointis pushed further away from the burner tip. We can then conclude thatthe oxygen injection velocity may be used to control the flame ignitionlocation and the main flame location. Also, the temperature in theoxygen enriched combustion zone is higher when the center O₂ injectionvelocity is doubled. This is important because the relatively hightemperature could increase the stability of the main flame. We can alsoconclude that the center O₂ injection velocity provides an adjustableparameter that can be tailored to the particular heating value of thebiomass being combusted in order to establish a robust, stable mainflame.

A comparison of Examples 2 and 8 show that if the velocity of the inner,annular wood pellet stream is doubled and the mass flow rate of theouter stream of wood pellets is decreased by a corresponding amount, therelationship between the velocities of the wood pellet and oxygenstreams is described by the equation: center oxygen velocity>inner,annular wood pellets velocity>outer, annular wood pellets velocity. Thishas the effect of increasing the mixing between the inner and outerannular streams of wood pellets and decreasing the mixing between theinner, annular stream of wood pellets and the central stream of oxygen.However, this reduces the oxygen enriched combustion zone—a conditionwhich is not preferred with respect to main flame ignition andstabilization.

Preferred processes and apparatus for practicing the present inventionhave been described. It will be understood and readily apparent to theskilled artisan that many changes and modifications may be made to theabove-described embodiments without departing from the spirit and thescope of the present invention. The foregoing is illustrative only andthat other embodiments of the integrated processes and apparatus may beemployed without departing from the true scope of the invention definedin the following claims.

What is claimed is:
 1. A biomass burner, comprising: a burner blockhaving an injector passage extending between rear and front faces; afuel conduit having inlet and outlet ends and being concentricallydisposed within said bore at said front face, an annular combustion airflow space being defined between an inner surface of said outer conduitand an outer surface of said fuel conduit; an oxygen injector havinginlet and outlet ends and being concentrically disposed within saidouter conduit at said front face; and a tubular fuel flow splitterconcentrically disposed within said fuel conduit at said front face,said splitter having an inlet end disposed upstream of said fuel conduitinlet end and also having an outlet end, wherein said oxygen injectorhas: an annular cross-sectional shape and is adjacent to and surroundssaid splitter; or a cylindrical cross-sectional shape and isconcentrically disposed within said splitter.
 2. The biomass burner ofclaim 1, wherein the outlet ends of the fuel conduit, oxygen injector,and splitter are flush with said front face.
 3. The biomass burner ofclaim 1, wherein the outlet ends of said oxygen injector and splitterare recessed back from said fuel conduit outlet end.
 4. The biomassburner of claim 1, wherein said oxygen injector has an annularcross-sectional shape and is adjacent to and surrounds said splitter. 5.The biomass burner of claim 4, wherein said oxygen injector outlet endis configured as a closed face with a plurality of radially distributedinjection holes.
 6. The biomass burner of claim 4, wherein said oxygeninjector outlet end is configured as an open face.
 7. The biomass burnerof claim 1, wherein said oxygen injector has a cylindricalcross-sectional shape and is concentrically disposed within saidsplitter.
 8. The biomass burner of claim 7, wherein said oxygen injectoroutlet end is configured as a closed face with a plurality of radiallydistributed injection holes.
 9. The biomass burner of claim 7, whereinsaid oxygen injector outlet end is configured as an open tube.
 10. Thebiomass burner of claim 1, further comprising an outer conduitconcentrically disposed within said bore having an inlet end disposeddownstream of said burner block rear face and also having an outlet end,said annular combustion air flow space being split into a secondarycombustion air flow space and a tertiary combustion flow space by saidouter conduit, the secondary combustion air flow space being defined byan outer surface of said fuel conduit and an inner surface of said outerconduit, the tertiary combustion air flow space being defined by anouter surface of said outer conduit and an inner surface of said bore.11. The biomass burner of claim 1, further comprising: a secondarycombustion air swirler disposed within said secondary combustion airflow space upstream of said burner block front face; and a tertiarycombustion air swirler disposed along an inner surface of said boreadjacent to said burner block front face.
 12. The biomass burner ofclaim 1, wherein said splitter further comprises a main sectionextending between said splitter inlet and outlet ends, the splitterinlet end having a diameter D1, the main body having a diameter D2,wherein D1<D2.
 13. The biomass burner of claim 1, wherein said fuelconduit has a diameter D4, said splitter inlet end has a diameter D1 and0.05 D4≦D1≦0.25 D4.
 14. A biomass combustion system, comprising thebiomass burner of claim 1, a biomass hopper, a biomass fuel feeder, asource of oxygen, and one or more blowers, wherein: said biomass fuelfeeder is operatively associated with said hopper and at least one ofsaid one or more blowers to receive particulate biomass from saidhopper, convey the particulate biomass with a flow of combustion airfrom said at least one blower to provide a flow of biomass fuel, anddirect the flow of biomass fuel to said fuel conduit inlet end; at leastone of said one or more blowers is in fluid communication with saidcombustion air flow space; and said source of oxygen is in fluidcommunication with said oxygen injector inlet end.
 15. The biomasscombustion system of claim 14, wherein said source of oxygen is selectedfrom group consisting of a vacuum swing adsorption system, an oxygenpipeline, a cryogenic air separation unit, and a vaporizer connected toa tank of liquid oxygen.
 16. A biomass-fired boiler installation,comprising: a plurality of the biomass burner of claim 1; one or moreblowers; at least one biomass hopper; at least one biomass fuel feeder;a source of oxygen; and a boiler, wherein said at least one biomass fuelfeeder is operatively associated with said at least one hopper and atleast one of said one or more blowers to receive particulate biomassfrom said at least one hopper, convey the particulate biomass with aflow of air from said at least one blower to provide a flow of biomassfuel, and direct the flow of biomass fuel to said fuel conduit inletends; at least one of said at least one blower is in fluid communicationwith said combustion air flow spaces; said source of oxygen is in fluidcommunication with said oxygen injector inlet ends; and said pluralityof burners are mounted on walls of said boiler.
 17. The biomass-firedboiler installation of claim 15, wherein said source of oxygen isselected from group consisting of a vacuum swing adsorption system, anoxygen pipeline, a cryogenic air separation unit, and a vaporizerconnected to a tank of liquid oxygen.
 18. A method of combustingbiomass, comprising the steps of: injecting a flow of particulatebiomass conveyed with air from a fuel conduit of a biomass burner into acombustion space, the fuel conduit having a tubular splitterconcentrically disposed therein, the flow of biomass being split by thesplitter into a central flow on the inside of the splitter and anannular flow on the outside of the splitter; injecting a flow of oxygeninto the flow of injected biomass from an oxygen injector concentricallydisposed within said fuel conduit; combusting the injected central flowof biomass with the oxygen in the combustion space; injecting an annularflow of combustion air from the burner around the annular flow ofbiomass; and combusting the injected annular flow of biomass with thecombustion air in the combustion space.
 19. The method of claim 18,wherein the central biomass flow has a velocity V1, the annular biomassflow has a velocity V2, and the flow of oxygen has a velocity V3, where(V3−V2)<(V3−V1).
 20. The method of claim 19, wherein no fuel other thanthe particulate biomass is combusted.
 21. The method of claim 19,wherein the oxygen and the central flow of biomass begin to mix at apoint upstream of said fuel conduit outlet end.
 22. The method of claim19, wherein said oxygen is injected in a center of the central flow ofbiomass.
 23. The method of claim 19, wherein said oxygen is injected inan annulus surrounding said splitter.
 24. The method of claim 19,wherein the oxygen is swirled.
 25. The method of claim 19, wherein theoxygen has a concentration of >95%.
 26. The method of claim 19, whereinthe combustion air is injected in two annular flows, a first of which issecondary combustion air adjacent the annular flow of biomass and asecond of which is tertiary combustion air adjacent the secondarycombustion air.
 27. The method of claim 19, wherein an overall oxygenenrichment of the combined biomass fuel, injected oxygen and combustionair achieved by injection of the oxygen is between 21% and 25%.